1. Field of the Invention
This invention relates in general to a structure of antifuse for semiconductor programmable logic devices and the process of its fabrication. In particular, this invention relates to an antifuse structure exhibiting asymmetric conductivity that allows for lower programming voltage and the process of its fabrication.
2. Description of Related Art
Antifuses are circuit elements for programmable logic devices that are the reverse of fuses. They are programmed switches in these logic devices that are normally open as opposed to the normal-closed switch status of fuses. When circuit voltage or current applied across both terminals of an antifuse reaches to certain level in programming sessions, these antifuses become short-circuited, and it becomes a close-circuited switch for the circuitry.
With reference to FIG. 1, in which a cross-sectional view schematically showing the structural configuration of a conventional antifuse is shown, it can be observed that a first electrically conductive layer 11, normally a polysilicon layer, covers the surface of the field oxide layer 10. An oxide insulation layer 12 covers the entire first conductive layer 11 and the surrounding region of the field oxide layer 10. An opening is formed in the designated region of this oxide insulation layer 12, revealing portions of the surface of the first conductive layer 11. Sidewall of this opening is sloped, reducing the size of the opening as it descends toward the bottom. After the formation of this opening, a thin dielectric layer 13 such as an NO is formed, covering the surface of the oxide insulation layer 12, including the sloped sidewall and the bottom of the opening. A second conductive layer 14 such as polysilicon is then formed, covering the dielectric layer 13. This generally concludes the fabrication of a conventional antifuse element for programmable logic devices.
The prior-art antifuse structure as briefly described in the structure of FIG. 1 is with some drawbacks. For example, if the programmable logic device employing this prior-art antifuse element for its programmable switch, then scaling down of the element structural configuration must be done in order to achieve feature improvements such as reduced operating voltage or improved low-resistance state. This would include measures such as reduction of the thickness of the dielectric layer 13. However, such measures would inevitably incur problems such as increased difficulties in the control over fabrication process conditions as well as increased possibility of defects.